Pepijn Veefkind

Retrieval of Aerosol Properties

Atmospheric aerosol is a suspension of liquid and solid particles in air, i.e. the aerosol includes both particles and its surrounding medium; in practice aerosol is usually referred to as the suspended matter, i.e. the particles or the droplets, depending on their aggregation state.

TROPOMI and TROPI: UV/VIS/NIR/SWIR instruments

TROPOMI (Tropospheric Ozone-Monitoring Instrument) is a five-channel UV-VIS-NIR-SWIR non-scanning nadir viewing imaging spectrometer that combines a wide swath (114°) with high spatial resolution (10 × 10 km2 ). The instrument heritage consists of GOME on ERS-2, SCIAMACHY on Envisat and, especially, OMI on EOS-Aura. TROPOMI has even smaller ground pixels than OMI-Aura but still exceeds OMIs signal-to-noise performance. These improvements optimize the possibility to retrieve tropospheric trace gases. In addition, the SWIR capabilities of TROPOMI are far better than SCIAMACHYs both in terms of spatial resolution and signal to noise performance. TROPOMI is part of the TRAQ payload, a mission proposed in response to ESAs EOEP call. The TRAQ mission will fly in a non-sun synchronous drifting orbit at about 720 km altitude providing nearly global coverage. TROPOMI measures in the UV-visible wavelength region (270-490 nm), in a near-infrared channel (NIR) in the 710-775 nm range and has a shortwave infrared channel (SWIR) near 2.3 μm. The wide swath angle, in combination with the drifting orbit, allows measuring a location up to 5 times a day at 1.5-hour intervals. The spectral resolution is about 0.45 nm for UVVIS- NIR and 0.25 nm for SWIR. Radiometric calibration will be maintained via solar irradiance measurements using various diffusers. The instrument will carry on-board calibration sources like LEDs and a white light source. Innovative aspects include the use of improved detectors in order to improve the radiation hardness and the spatial sampling capabilities. Column densities of trace gases (NO2, O3, SO2 and HCHO) will be derived using primarily the Differential Optical Absorption Spectroscopy (DOAS) method. The NIR channel serves to obtain information on clouds and the aerosol height distribution that is needed for tropospheric retrievals. A trade-off study will be conducted whether the SWIR channel, included to determine column densities of CO and CH4, will be incorporated in TROPOMI or in the Fourier Transform Spectrometer SIFTI on TRAQ. The TROPI instrument is similar to the complete TROPOMI instrument (UV-VIS-NIR-SWIR) and is proposed for the CAMEO initiative, as described for the U.S. NRC Decadal Study on Earth Science and Applications from Space. CAMEO also uses a non-synchronous drifting orbit, but at a higher altitude (around 1500 km). The TROPI instrument design is a modification of the TROPOMI design to achieve identical coverage and ground pixel sizes from a higher altitude. In this paper capabilities of TROPOMI and TROPI are discussed with emphasis on the UV-VIS-NIR channels as the TROPOMI SWIR channel is described in a separate contribution [5].

TROPOMI: Solar backscatter satellite instrument for air quality and climate

TROPOMI is a nadir-viewing grating-based imaging spectrograph in the line of OMI and SCIAMACHY. TROPOMI is part of the ESA Candidate Core Explorer Mission proposal TRAQ and also of the CAMEO satellite proposed for the US NRC decadal study. A TROPOMI-like instrument is part of the ESA/EU Sentinel 4&5 pre-phase A studies. TROPOMI covers the OMI wavelengths of 270-490 nm to measure O3, NO2, HCHO, SO2 and aerosols and adds a NIR channel and a SWIR module. The NIR-channel (710-775 nm) is used for improved cloud detection and aerosol height distribution. The SWIR module (2305 - 2385 nm) measures CO and CH4 and forms a separate module because of its thermal requirements. TROPOMI is a non-scanning instrument with an OMI-like telescope but optimized to have smaller ground pixels (10 x 10 km2) and sufficient signal-to-noise for dark scenes (albedo 2 %). TROPOMI has the same wide swath as OMI (2600 km). In TRAQs mid-inclination orbit, this allows up to 5 daytime observations over mid-latitude regions (Europe, North-America, China). The paper gives a description of the TROPOMI instrument and focuses on several important aspects of the design, for example the sun calibration and detector selection status.

Pre-launch calibration results of the TROPOMI payload on-boardthe Sentinel 5 Precursor satellite

The Sentinel-5 Precursor satellite was successfully launched on 13 October 2017, carrying the Tropospheric Monitoring Instrument (TROPOMI) as its single payload. TROPOMI is the next generation atmospheric sounding instrument, continuing the successes of GOME, SCIAMACHY, OMI and OMPs, with higher spatial resolution, improved sensitivity and extended wavelength range. The instrument contains four spectrometers, divided over two modules sharing a common telescope, measuring the ultraviolet, visible, near-infrared and shortwave infrared reflectance of the Earth. The imaging system 5 enables daily global coverage using a push-broom configuration, with a spatial resolution as low as 7 x 3.5 km in nadir from a Sun-synchronous orbit at 824 km and an equator crossing time of 13:30 local solar time. This article reports the pre-launch calibration status of the TROPOMI payload as derived from the on-ground calibration effort. Stringent requirements are imposed on the quality of on-ground calibration in order to match the high sensitivity of the instrument. 10 A new methodology has been employed during the analysis of the obtained calibration measurements to ensure the consistency and validity of the calibration. This was achieved by using the production grade Level 0 to 1b data processor in a closed-loop validation setup. Using this approach the consistency between the calibration and the L1b product could be established, as well as confidence in the obtained calibration result. This article introduces this novel calibration approach, and describes all relevant calibrated instrument properties as they 15 were derived before launch of the mission. For most of the relevant properties compliance with the calibration requirements could be established, including the knowledge of the instrument spectral and spatial response functions. Partial compliance was established for the straylight correction; especially the out-of-spectral-band correction for the NIR channel needs future validation. The absolute radiometric calibration of the radiance and irradiance responsivity is compliant with the high level mission requirements, but not with the stricter calibration requirements as the available on-ground validation shows. The 20

TROPOMI, the solar backscatter satellite instrument for air quality and climate, heads towards detailed design

The Tropospheric Monitoring Instrument (TROPOMI) is currently planned for launch on ESAs Sentinel 5 precursor satellite in the time frame of 2014. TROPOMI is an ultraviolet-to-SWIR wavelengths imaging spectrograph that uses two-dimensional detectors to register both the spectrum and the swath perpendicular to the flight direction. The swath is about 110 degrees wide to allow daily global coverage from the polar orbit of the Sentinel 5 precursor satellite. The instrument follows the heritage of SCIAMACHY (ENVISAT, launch 2002) and OMI (AURA, launch 2004), but it has been improved in several ways: the ground resolution is down to 7 x 7 km2, the instrument is fit for low albedo scenes and the wavelength bands are optimized using the SCIAMACHY and OMI heritages to have the best trace gas products. The first two improvements basically mean that the instrument aperture is much larger for TROPOMI and, related to this, the reading of the detectors much faster. The selected wavelength bands for TROPOMI are UV1 (270-310 nm), UV2 (310 - 370 nm), VIS (370 - 500 nm), NIR (675 - 775 nm) and SWIR (2305 - 2385 nm). The first three bands are very similar to the OMI bands, the NIR has been added to improve on clouds and air mass corrections and the SWIR allows measuring CH4 and CO. The paper discusses the development status on several topics, such as detector selection and polarization scrambler performance simulations using the TIDE grid based level 2 scene simulator.

Satellite and ship-based lidar measurements of optical depth during EOPACE

The authors consider methods for obtaining quantitative data of the coastal marine atmospheric boundary layer (MABL). Satellite remote sensing is the only data source that can measure MABL properties in the coastal zone with the needed high spatial resolution. However, many of the uses of satellite data are qualitative. Quantitative satellite remote sensing methods need to be tested to provide littoral data. Ship-based aerosol and lidar backscatter measurements, aircraft-measured aerosol, and rawinsonde data obtained during the Electro-Optical Propagation Assessment in Coastal Environments (EOPACE) Intense Observing Periods (IOPs) provide an opportunity to compare satellite methods with in situ data. The purpose of the EOPACE program is to characterize the aerosol and boundary layer properties in the coastal zone and to determine if air mass parameters in various coastal locations can be derived, to a practical degree, from satellite imagery. The combination of satellite data with several in situ surface and aircraft data sets offers an excellent opportunity to monitor significant optical depth and aerosol changes in the coastal zone. In addition, quantitative comparisons can be made. The objective of this paper is to evaluate satellite-derived aerosol optical depths estimates using aircraft and ship-based aerosol measurements, a ship-based lidar and rawinsonde profiles of the MABL. Results from the April 1996 IOP are presented.

TROPOMI on ESA’s Sentinel 5p ready for launch and use

TROPOMI is the single instrument on ESA’s Sentinel 5 precursor satellite to be launched in October 2016. TROPOMI will measure the atmospheric constituents absorbing in the UV-SWIR wavelength range, being O3, NO2, SO2, CH4, CO, CH2O, and aerosol properties. TROPOMI is a sun back-scatter instrument in the line of SCIAMACHY and OMI with 4 spectrometer bands and a spectral resolution of 0.25 – 0.5 nm. Following the earlier sensors, firstly the spatial resolution is improved by a factor 6 (OMI) to 7 x 7 km2 and at the same time the sensitivity by an order of magnitude. The paper discusses the instrument performances as acquired from on-ground performance / calibration measurements. For the calibration an extremely condensed measurement campaign of 4 months 24/7 measurements was performed with virtually no slack and still gathering all of the data necessary from on-ground measurements. Given the fact that the trace gas signals and their variation in the measured spectra can be quite small, calibration is crucial to get accurate results and this illustrates that TROPOMI is a highly success driven and efficient programme. TROPOMI / Sentinel-5p bridges the data streams from on one hand OMI and SCIAMACHY and on the other hand the future Sentinel-5. It is the first of a series of satellites from the Copernicus programme devoted to air quality and will soon be ready for use.

Retrieval of aerosol optical depth from satellite measurements using single and dual view algorithms

To retrieve aerosol properties from the radiance measured at the top of the atmosphere in clear sky conditions, the contributions of the surface and the various atmospheric constituents need to be separated. This is easiest done over dark surfaces, i.e. with a negligible reflectance that can relatively easily be accounted for. This principle has been used for the retrieval of aerosol optical depth (AOD) over water from satellite observations in the near-infrared. The AOD at wavelengths in the UV can be determined both over water and over land, using the same principle of a dark surface. For longer wavelengths, the dual view provided by ATSR-2 is used to separate the aerosol reflectance from the surface contribution. Single and dual-view algorithms have been developed by TNO-FEL and tested for the US east coast and over Europe. Currently the algorithms are extended with other aerosol types and tested versus data over the Indian Ocean (INDOEX area) and South Africa (SAFARI experiment). The initial results indicate that the AOD can be retrieved within reasonable limits. Apart from the ATSR-2, algorithms aimed at the determination of aerosol optical depth and composition are developed for AATSR and SCIAMACHY (ENVISAT) and OMI (EOS-TERRA).

The Tropomi instrument: last steps towards final integration and testing

The Tropospheric Monitoring Instrument TROPOMI is ready for system level verification. All sub-units have been integrated and tested and final integration at Dutch Space in Leiden has been completed. The instrument will be subjected to a testing and calibration program and is expected to be ready for delivery to the spacecraft early 2015. Using TROPOMI measurements, scientists will be able to improve and continue the study of the Earth’s atmosphere and to monitor air quality, on both global and local scale.

From ozone monitoring instrument (OMI) to tropospheric monitoring instrument (TROPOMI)

The OMI instrument is an ultraviolet-visible imaging spectrograph that uses two-dimensional CCD detectors to register both the spectrum and the swath perpendicular to the flight direction with a 115° wide swath, which enables global daily ground coverage with high spatial resolution. This paper presents a selection of in-flight radiometric and CCD detector calibration and performance monitoring results since the launch in July 2004. From these examples it will be shown that OMI is performing very well after more than four years in orbit. It is shown how the OMI irradiance measurement data have been used to derive a high resolution solar reference spectrum with good radiometric calibration, good wavelength calibration and high spectral sampling. The surface reflectance climatology derived from three years of in-orbit OMI measurement data is presented and discussed. The OMI mission may possibly be extended in 2009 for another two or four years, depending on the performance of the instrument. By 2013-2014 OMI on EOS-Aura and SCIAMACHY on ENVISAT will have reached more that twice their anticipated lifetimes. In order to guarantee continuity of Earth atmosphere tropospheric and climate measurement data new instrumentation shall be available around that time. A successor of OMI and SCIAMACHY, named TROPOspheric Monitoring Instrument (TROPOMI), scheduled for launch by the end of 2013, is discussed in this paper.